Liquid gaging system lost sensor recovery

ABSTRACT

The present invention is part of a liquid gaging system and comprises an apparatus for reducing error introduced by an incapacitated sensor. Included are a plurality of sensors, each sensor providing a liquid measurement signal relating to a liquid depth at a particular location in a tank. The invention also comprises apparatus for selectively monitoring the liquid measurement signal of each individual sensor, the apparatus for selectively monitoring being connected to the sensors. Also included is apparatus for identifying at least one sister sensor corresponding to each sensor. The invention further comprises apparatus for determining an estimated liquid depth corresponding to an incapacitated sensor by using the liquid measurement signal of at least one sister sensor corresponding to the incapacitated sensor. The apparatus for determining is connected to the apparatus for selectively measuring and the apparatus for identifying.

CROSS REFERENCES TO RELATED APPLICATIONS

Reference is made to a co-pending application entitled "Liquid GagingSystem" by W. R. Dougherty, D. D. Colby, and M. J. van Dyke, Ser. No.036,119, filed May 4, 1979, and assigned to the same assignee as thisapplication (now U.S. Pat. No. 4,258,422).

Reference is also made to a co-pending application entitled "LowRadiation Densitometer" by R. J. Borken, Ser. No. 081,962, filed Oct. 4,1979, and assigned to the same assignee as this application (now U.S.Pat. No. 4,277,681).

Reference is further made to the following co-pending applications whichwere filed on May 14, 1980 and are assigned to the same assignee as thisapplication:

"Liquid Gaging System Compatible With Multiple Characterization of EachSensor" by M. J. van Dyke, D. D. Colby, and W. R. Dougherty, Ser. No.149,795

"Digital Characterization of Liquid Gaging System Sensors" by D. D.Colby, M. J. van Dyke, and W. R. Dougherty, Ser. No. 149,772

"Liquid Gaging System Null Balance Circuitry" by M. J. van Dyke and J.A. Fahley, Ser. No. 149,789

"Liquid Gaging System Multiplexing" by W. R. Dougherty, D. D. Colby, J.A. Fahley, and M. J. van Dyke, Ser. No. 149,798

"Liquid Gaging System Contamination Monitor" by J. A. Fahley, Ser. No.149,790

"Liquid Gaging System Self Test Circuitry" by K. Leonard, D. D. Colby,W. R. Dougherty, J. A. Fahley, and M. J. van Dyke, Ser. No. 149,797

"Liquid Gaging System Sensor Calibration" by W. R. Dougherty, Ser. No.149,796.

BACKGROUND OF THE INVENTION

The present invention relates to a microcomputer-controlled system formeasuring the volume or quantity of liquid in one or more tanks.Although the present invention has application in a variety of liquidgaging systems, it will be described in the context of an aircraft fuelgaging system.

In the aircraft industry, a basic sensor for measuring fuel volume orquantity has long been the capacitance sensor which has been acceptedfor many years as a rugged, reliable device. In the above mentionedco-pending application filed on May 4, 1979 (application Ser. No.036,119), now U.S. Pat. No. 4,258,422, a system was described whichprovided significant improvement in the sensor accuracy of liquid gagingsystems, including systems for measuring aircraft fuel volume orquantity with capacitive sensors. As with the present system, the systemdescribed in application Ser. No. 036,119 achieves improved sensorgaging accuracy and flexibility by use of a microcomputer or similardevice to provide tank shape and volume, tank or aircraft attitude, andsimilar characterization which in the prior art was formerly onlyapproximated by means of physically characterized (shaped) fuel gageprobes.

In this manner, like the liquid gaging system described in applicationSer. No. 036,119, now U.S. Pat. No. 4,258,422 the present systemprovides a number of significant advantages over conventional liquidgaging systems. These advantages include the need for a fewer number ofsensors or probes in each tank, simplified probe construction byelimination of physical characterization, improved system accuracy bycharacterizing for tank geometry and tank or airplane attitude in amicrocomputer or other digital system, reduced system weight bydecreasing the number of probes, and simplified installation for theaircraft manufacturer by requiring fewer probes. Digitalcharacterization also provides a more flexible design which canaccommodate tank changes with minor hardware impacts.

The present system was developed to provide a liquid gaging systemhaving even greater accuracy and flexibility than previously disclosedsystems. One feature of the present system providing significantadditional accuracy and flexibility is the individual measurement of thewetted length of each probe. This is in contrast to typical prior artsystems and the system described in application Ser. No. 036,119 nowU.S. Pat. No. 4,258,422 wherein the total wetted length of all probes ina particular tank is measured and used for determination of liquidvolume or quantity. Thus, in the system described in application Ser.No. 036,119, now U.S. Pat. No. 4,258,422 a wetted length signal relatedto the portion of all probes in a tank wetted by the liquid is used inconjunction with data stored in attitude tables for determining liquidvolume or quantity in a tank.

In contrast, one advantage of the present system is that no attitudesensor is required, and no signal need be received by the system toseparately indicate the attitude of the tank or aircraft. Elimination ofan attitude sensor is made possible in part through the previouslymentioned individual monitoring of the wetted length of each probe.Knowing the individual wetted length of each probe, the determination offuel volume or quantity can be made through stored data having theeffects of attitude, tank geometry, the number and location of theprobes, and similar factors already included. Thus, the present systemprovides the basis for significant simplification over systems requiringattitude sensors since attitude sensors with sufficient precision andreliability are complex and expensive. Accordingly, the present systemprovides the basis for improving reliability and reducing maintenancerequirements.

The present system also provides significantly increased accuracy overthe accuracy of systems available in the prior art. In the preferredembodiment, each probe in the present system is divided into theoreticalsections having a length, and at least one characterization tablecorresponding to each probe is used with data related to the theoreticalsections and the wetted length of the probe to directly determine fuelvolume or quantity. By having a plurality of characterization tablescorresponding to each probe, multiple characterizations governingvarious conditions can be used. Thus, for example, there can be separatecharacterizations for ground and flight conditions and for separateranges of liquid volume or quantity. As will be further discussed inthis application, such multiple characterization of each probe cansignificantly increase system accuracy.

In addition, prior art systems compatible with individual monitoring ofphysically characterized probes can be retrofitted with the presentsystem in order to provide multiple characterization of each probe,thereby significantly increasing the accuracy of the such systems.

The present system also incorporates digital null balance circuitry anda rebalance approximation sequence which provides a high-speed readingmeans for very quickly and accurately determining the capacitance ofeach sensor as the sensors are individually monitored. The digital nullbalance circuitry in the present system is in contrast to relativelyslow prior art circuitry using means such as up-down counters to monitora reading. A high-speed accurate system is of substantial advantagesince, for example, in an aircraft fuel gaging system, a relativelyshort probe could very quickly go from being 100 percent wetted to beingcompletely unwetted. In such a situation, the relatively slow prior artreading means could be incompatible with multiplexed probe readings suchas those in the present system.

Another advantage of the present system is its ability to monitor orfault isolate various components for malfunction. For example, through acontamination monitor and related circuitry, each probe and dielectricsensor may be monitored for two levels of contamination, a first levelindicating that the component is becoming contaminated but is stilluseable and a second level indicating the component is no longer useful.

Another example of the faul isolation capabilities of the present systemrelates to two precision reference components, a reference resistor anda reference capacitor, each of which is driven by the system excitationsignal generator. By monitoring the current through the referenceresistor using the null balance circuitry, a primary check of theexcitation signal level is obtained, and a secondary check of the nulldetecting circuitry is made. By monitoring the excitation signal throughthe reference resistor via the contamination monitor, a primary check ofthe contamination monitor is made, and a secondary check of theexcitation signal level is obtained. By monitoring the current throughthe reference capacitor, a primary check of the excitation signalfrequency is obtained, and a secondary check of the null detectingcircuitry is made.

A still further advantage of the present system relates to eliminationof a significant portion of the additional error introduced by completeloss of a probe such as when a single capacitive probe is disabled andprovides a zero capacitance reading or when a probe is determinedunusable by the contamination monitor. In a preferred embodiment of thepresent system, one or more sister probes are identified for each probeand used to estimate a failed probe's wetted sensing length. In thismanner, if a probe does malfunction completely, substantially lessprecision is lost than in typical prior art systems under suchcircumstances.

An even further advantage of the present system is that, sinceindividual probes are monitored for wetted sensing length, individualcapacitive probes can be monitored while unwetted in order to determinetheir true unwetted capacitance, including any stray capacitanceintroduced during installation. Combined with a storage device such asan electrically programmable read-only memory (EPROM), monitoringindividual capacitive probes while unwetted allows exact emptycapacitances to be stored in the microcomputer for use in makingextremely precise liquid volume or quantity determinations. The featureis particularly applicable upon installation of a system or after anoverhaul during which additional or different stray capacitances mayhave been introduced.

SUMMARY OF THE INVENTION

The present invention is part of a liquid gaging system and comprises anapparatus for reducing error introduced by an incapacitated sensor.Included are a plurality of sensors, each sensor providing a liquidmeasurement signal relating to a liquid depth at a particular locationin a tank. The invention also comprises apparatus for selectivelymonitoring the liquid measurement signal of each individual sensor, theapparatus for selectively monitoring being connected to the sensors.Also included is apparatus for identifying at least one sister sensorcorresponding to each sensor. The invention further comprises apparatusfor determining an estimated liquid depth corresponding to anincapacitated sensor by using the liquid measurement signal of at leastone sister sensor corresponding to the incapacitated sensor. Theapparatus for determining is connected to the apparatus for selectivelymeasuring and the apparatus for identifying.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a liquid gaging system comprising thepresent invention.

FIGS. 2A and 2B comprise a block diagram of analog-to-digital (A/D)electronics compatible with the present invention.

FIGS. 3A through 3E are schematic diagrams of the A/D electronics shownin FIG. 2.

FIG. 3F is a block diagram of a microcomputer compatible with thepresent system.

FIG. 4 illustrates various waveforms associated with the operation ofthe A/D electronics shown in FIGS. 3A through 3E.

FIGS. 5A and 5B illustrate a physically uncharacterized probe compatiblewith the present system.

FIGS. 6A and 6B illustrate a dielectric sensor compatible with thepresent system.

FIG. 7 compares six digital characterizations of a probe with a best fitfixed or physical characterization.

FIG. 8 facilitates explanation of fuel volume determination in thepresent system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS System Overview

As was previously indicated, the present invention relates to amicrocomputer-controlled system for measuring the liquid volume orquantity in one or more tanks. The present application describes theentire electrical system and all of the various functions of the systemin order to permit a complete appreciation of the various functions andactivities within the system. As a result, there are described in thisapplication various features and functions of the liquid gaging systemwhich are not the subject of the present invention but rather are thesubject of inventions claimed in the previously mentioned co-pendingapplications filed concurrently with this application. The descriptionof these features and functions are included in the present applicationfor the sake of completeness and to permit the reader a fullappreciation of the operation of the liquid gaging system disclosed.

The embodiment of the present system illustrated in FIG. 1 comprises asection of analog-to-digital (A/D) electronics 24, a microcomputer 25,and three tanks which, for example, could comprise a left wing tank 21L,a center tank 21C, and a right wing tank 21R of an aircraft.

For each tank to be monitored, the liquid gaging system includes one ormore probes 22, each probe providing a wetted sensor length signalvariable in dependence on the portion of the probe immersed in liquid.Probes 22 may be of the capacitive type or may be any type of probecompatible with the present invention. In the alternative, probes 22 maybe replaced by any type of sensor for providing information relative toa liquid level or depth at a particular location in a tank. A pluralityof probes for a tank may be referred to as a probe system. Likewise, aplurality of sensors for a tank may be referred to as a sensor system.

Capacitive probes typically have a capacitance variable in dependence onthe portion of the probe immersed in liquid and on the dielectricconstant of the liquid. Consequently, if capacitive probes are used, asystem incorporating the present invention would normally comprise atleast one dielectric compensator or sensor 23 for measuring thedielectric constant of the liquid. The system shown includes adielectric sensor 23 in each tank since the liquid in various tanks mayvary in dielectric constant due, for example, to differences in liquidmixture or temperature.

A primary purpose of A/D electronics 24 is to convert an analog signalfrom each probe 22 and dielectric sensor 23 into an accurate binaryword. In the case of each probe 22, the binary word in the systemdisclosed represents a capacitance added to the probe by the liquid. Inthe case of each dielectric sensor 23, the binary word is related to thedielectric constant of the liquid.

The functions of microcomputer 25 include controlling A/D electronics 24and, together with A/D electronics 24, generating the binary numbersrepresenting liquid volume or quantity in the tank or tanks of interest.Although the present system is controlled almost exclusively through amicrocomputer 25, it is also possible to provide control throughsequencers or other electronics. Microcomputer 25 also serves as meansfor receiving signals or data from an aircraft air/ground relay 28, thesignals or data being related to whether the aircraft is in the air oron the ground.

Readout of fuel volume or quantity in each tank or within all tanks maybe provided through conventional digital drive electronics andindicators (not shown).

Depending upon the requirements of a particular liquid gaging system, itmay be desirable to include more than one channel of A/D electronics 24and/or more than one microcomputer 25. Thus, a liquid gaging systemcould comprise two or more channels of A/D electronics 24 and/ormicrocomputers 25. In such a system, the channels can be redundant, eachperforming the same task, one serving as backup for the other.Alternatively, each channel can simultaneously perform completelyindependent functions or measurements, with the further operation ofeach channel being available as backup for the other(s) underpredetermined conditions of failure modes.

In operation of the present system as disclosed, probes and dielectricsensors are sequentially scanned in order to determine the capacitanceof each probe and the dielectric constant of the fuel.

Using the measured probe capacitances and the value of the fueldielectric constant, a value of wetted sensing length is determined foreach probe. The wetted sensing length of each probe is then converted toa measure of partial fuel volume corresponding to the probe. The partialfuel volumes computed for each probe are then summed to obtain a totalfuel volume for each tank.

If fuel quantity in weight such as pounds is desired as opposed to fuelvolume such as gallons, fuel density may be determined through themeasured dielectric constant, as further discussed elsewhere in thisapplication. Alternatively, fuel density may be determined by any otherdensity sensor meeting system accuracy requirements. Total fuel quantityin each tank may then be calculated by multiplying fuel volume timesfuel density for each tank. Total fuel quantity on board the aircraftmay then be computed from the sum of the individual tank fuel quantitydeterminations.

A/D Electronics Overview

A preferred embodiment of A/D electronics 24 is illustrated in the blockdiagram of FIGS. 2A and 2B and is further detailed in the schematicdiagram of FIGS. 3A through 3F. In these figures, like-numberedterminals, e.g., I1 in FIGS. 2A and 2B, are connected.

Referring to FIGS. 2A and 2B, it can be seen that A/D electronics 24comprises an excitation signal generator 26, an excitation multiplexer44, and an input multiplexer 60. Excitation signal generator 26 providesan excitation signal to various components including probes 22 anddielectric sensors 23. Excitation multiplexer 44 controls application ofthe excitation signal to probes 22 and dielectric sensors 23. Inputmultiplexer 60 controls selection of various measurements, includingliquid measurements related to the individual tanks.

A/D electronics 24 also comprises a null threshold detector 71, emptyreference circuitry 34, and full reference circuitry 36. Thesecomponents together with associated circuitry are used to make varioussystem measurements through digital null balancing.

Also included are quadrature filter drivers 40, a quadrature filter 66,and a contamination monitor 67. Together with drivers 40, quadraturefilter 66 filters the quadrature or resistive component of each liquidmeasurement signal in order to measure the true capacitive value of thesignal. Contamination monitor 67 checks probe and dielectric sensorcontamination and can be used to determine if the amplitude of theexcitation signal provided by excitation signal generator 26 is above apredetermined level.

A key feature of the present system is that each probe 22 and eachdielectric sensor 23 is individually monitored. In any one channel ofthe system shown, this individual monitoring is accomplished throughsimultaneous excitation of one probe 22 or dielectric sensor 23 in eachtank while at the same time monitoring only one tank at a time forliquid measurement.

In the preferred embodiment, individual probes 22 and dielectric sensors23 are selected for measurement by means of excitation multiplexer 44and input multiplexer 60. Each of these multiplexers is controlled bysignals received through a plurality of second inputs, numbers 48, 48A,and 48B corresponding to excitation multiplexer 44 and numbers 59, 59A,and 59B corresponding to input multiplexer 60. In any one channel of theA/D electronics 24 preferred embodiment, excitation multiplexer 44switches the excitation signal provided by excitation signal generator26 to the low impedance electrode of one probe 22 or dielectric sensor23 in each tank or to load control select 52. (Load control select 52relates to control of liquid quantity to be put in a tank and does notdirectly relate to the present invention. Accordingly, it will not bediscussed in detail in this application). Excitation multiplexer 44 alsosimultaneously grounds all low impedance electrodes not receiving theexcitation signal. Lines 51, 53, and 54 leading from multiplexer 44 arelabeled with a slash (/) and a corresponding number (e.g. 14) toindicate the number of lines involved. For example, the slash and number14 on lines 53 indicate 14 lines, each line leading to one of 14 probes22 in each tank. Similarly, the slash and number one (1) associated withline 54 indicates one line leading to a dielectric sensor 23 in eachtank. Likewise, the slash and number one associated with line 51indicates one line leading to load control select 52. Similarnomenclature is used elsewhere in this application (e.g., see FIG. 3F).

The high impedance electrode of each probe 22 and dielectric sensor 23are commoned for each tank and connected to an input amplifier for eachtank. These amplifiers, numbers 56L, 56C, and 56R corresponding to tanks21L, 21C, and 21R respectively, function as current to voltageconverters. Each amplifier 56L, 56C, and 56R has its own connection toinput multiplexer 60 which individually selects one amplifier at a timefor liquid measurement.

Obtaining a liquid measurement in a particular tank of interest relatesto obtaining a null at the input of the amplifier corresponding to thetank. Thus, if a measurement is being made on a probe 22 in tank 21L, anull is obtained at input 55L of amplifier 56L. Similarly, if ameasurement is being made through amplifier 56C or 56R, a null isobtained at input 55C or 55R, respectively. As indicated above, and aswill be further explained below, these nulls are detected throughcircuitry comprising null threshold detector 71, empty referencecircuitry 34, and full reference circuitry 36.

In understanding the operation of the null detecting and associatedcircuitry, it may be convenient to think of the capacitance of aparticular probe 22 or dielectric sensor 23 as comprising two separatevalues, a first value related to the dry or unwetted capacitance of theprobe or sensor and a second value related to the capacitance added tothe probe or sensor by the liquid.

In balancing the current provided by a particular probe 22 or dielectricsensor 23, the circuitry of the embodiment illustrated separatelyprovides a current to balance the current associated with the dry orunwetted capacitance of the probe or dielectric sensor and a current tobalance the current associated with the capacitance added by the liquidto the probe or dielectric sensor.

Accordingly, empty reference circuitry 34 provides the current tobalance the dry or unwetted capacitance of the probe 22 or dielectricsensor 23 of interest. The current provided by empty reference circuitry34 is provided through an empty reference capacitor corresponding to theparticular tank or amplifier associated with the probe 22 or dielectricsensor 23 being monitored.

Similarly, during the monitoring of a particular probe 22 or dielectricsensor 23, full reference circuitry 36 provides current to balance thecurrent associated with the capacitance added to the probe or dielectricsensor by the liquid. During such a measurement, current provided byfull reference circuitry 36 is provided through a full referencecapacitor corresponding to the particular tank or amplifier associatedwith the probe or dielectric sensor being monitored.

Empty Reference circuitry 34

As was previously indicated, the purpose of empty reference circuitry 34is to provide a particular current to the input of an amplifiercorresponding to the tank containing the probe 22 or dielectric sensor23 of interest. The current provided will balance the current associatedwith the dry or unwetted capacitance of the probe or dielectric sensorbeing monitored.

Empty set circuitry 34 comprises an amplifier 72 driven by excitationsignal generator 26. Within circuitry 34, an output 72A of amplifier 70is coupled to an empty set digital-to-analog (D/A) converter 73 which,together with an amplifier 79 (FIG. 3A), provides at an output 79A avoltage that is opposite in phase to that provided by output 30 ofgenerator 26. Circuitry 34 also includes three empty adjustpotentiometers 74, 75, and 76. These potentiometers could be replaced inappropriate circumstances by fixed voltage dividers comprisingresistors.

Amplifier 72, which may comprise an LF156, serves to provide an emptyreference voltage to D/A converter 73. In the embodiment shown, D/Aconverter 73 is a ten-bit unit and may comprise a 7520UD. Amplifier 79,which may also comprise an LF156, is used with D/A converter 73 toprovide a voltage output since a 7520UD does not include an amplifier onthe chip. Converter 73 provides empty set attenuation means and includesa plurality of second inputs 87 for receiving empty set or attenuationcontrol signals from microcomputer 25. Converter 73 providespredetermined attenuation dependent upon which second inputs 87 areenabled by the attenuation control signals.

When the most significant bit (MSB) of converter 73 is enabled, theconverter passes half of the input signal. For a 10 bit converter, eachbit of lesser significance, when enabled by itself, permits converter 73to pass 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, and 1/1024respectively of the current entering the converter. Thus, if the firstthree most significant bits are enabled, 87.5% of the current is passed(1/2+1/4+1/8=0.875), and if all bits are enabled, substantially allcurrent passes. Therefore, by enabling appropriate inputs of converter73, predetermined, highly precise attenuation factors can be selected.

Empty adjust potentiometers 74, 75, and 76 correspond to tanks 21L, 21C,and 21R respectively and to three empty reference capacitors 80, 82, and84 respectively. The current provided by empty reference circuitry 34 isprovided through an empty reference capacitor corresponding to aparticular tank.

As is explained further below, when a particular tank is being monitoredfor liquid measurement, the current flowing through the appropriateempty reference capacitor is set to be equal in magnitude but oppositein phase to the current flowing through the unwetted probe 22 ordielectric sensor 23 of interest.

During initial set up of a system, each empty adjust potentiometer (orother suitable voltage divider) is set (sized) to balance the currentflowing through its corresponding empty reference capacitor with thecurrent flowing through the largest capacitance probe 22 in thecorresponding tank while the probe is dry or unwetted.

The adjustment is accomplished using a product of voltage andcapacitance, the product being proportional to current. Each emptyadjustment potentiometer is set so that the product of the emptyreference voltage provided by amplifier 72 and the capacitance of thecorresponding empty reference capacitor is equal to the product of theexcitation signal voltage provided by excitation signal generator 26 andthe empty or unwetted capacitance of the corresponding largest probe 22.

Accordingly, empty adjust potentiometer 74 is set so that the product ofthe empty reference voltage provided by amplifier 72 and the capacitanceof empty reference capacitor 80 is equal to the product of theexcitation signal voltage provided by excitation signal generator 26 andthe empty or unwetted capacitance of the largest probe 22 in tank 21L.Similarly, empty adjust potentiometers 75 and 76 are set so that theproduct of the empty reference voltage and the capacitance of emptyreference capacitors 82 and 84 respectively are equal to the product ofthe excitation signal voltage and the empty or unwetted capacitance ofthe largest probe 22 in tanks 21C and 21R respectively.

Resistors 74A, 75A, and 76A serve as voltage dividers so that the entirevoltage drop is not accomplished by the resistors in potentiometers 74,75, and 76. In this manner a greater sensitivity to the potentiometeradjustments is provided.

Thermistors 74B, 75B, and 76B (FIG. 3A) provide compensation to offsetthe fixed temperature coefficients of empty reference capacitors 80, 82,and 84 respectively.

Whenever input multiplexer 60 selects the largest probe 22 in a tank formeasurement, all second inputs 87 of empty set D/A converter 73 areenabled so that converter 73 provides the empty reference voltagewithout substantial attenuation. During such a measurement, the currentflowing through the corresponding empty reference capacitor balances thecurrent that would flow through the corresponding largest probe whileunwetted.

However, if a measurement is being made on a probe 22 other than a probehaving the largest unwetted capacitance or on an unwetted dielectricsensor 23, the current flowing through the corresponding empty referencecapacitor will be larger than the current corresponding to the unwettedcapacitance of the probe or sensor unless an appropriate attenuation isprovided by empty set D/A converter 73. Thus, whenever such ameasurement is selected, microcomputer 25 provides appropriateattenuation control signals to second inputs 87, there being stored in amemory of microcomputer 25 an empty set attenuation factor for eachprobe and dielectric sensor. With the appropriate attenuation enabled,the current provided to the corresponding input amplifier by theapplicable empty reference capacitor will be the current correspondingto the unwetted capacitance of the probe 22 or dielectric sensor 23being monitored.

Full Reference Circuitry 36

As was previously indicated, the purpose of full reference circuitry 36is to provide a particular balancing current to the input of anamplifier of interest.

Full reference circuitry 36 comprises an amplifier 38 driven byexcitation signal generator 26. Circuitry 34 also comprises a full setdigital-to-analog (D/A) converter 90, an amplifier 98, a rebalancedigital-to-analog (D/A) converter 91, an amplifier 99, and three fulladjust potentiometers 94, 95, and 96. As with the potentiometers inempty reference circuitry 34, potentiometers 94, 95, and 96 could bereplaced by suitable fixed voltage dividers.

Amplifier 38 may comprise an LF156 and serves as an inverting fullreference amplifier means to generate a voltage that is opposite inphase to that provided by output 30 of excitation signal generator 26.

In the embodiment shown, D/A converters 90 and 91 may be 10 bit units,and each converter may comprise a 7520UD. Amplifiers 98 and 99, each ofwhich may comprise an LF156, are used in conjunction with converters 90and 91 respectively to provide a voltage output since 7520UD chips donot include an amplifier.

Converter 90 provides full set attenuation means and includes aplurality of second inputs 92 for receiving full set or attenuationcontrol signals from microcomputer 25. Converter 90 providespredetermined attenuation dependent upon which second inputs 92 areenabled by the attenuation control signals.

Converter 91 provides reference means and includes a plurality of secondinputs 93 for receiving rebalance approximation sequence control signals(sometimes termed rebalance control signals) from microcomputer 25. In amanner similar to converter 90, converter 91 provides predeterminedattenuation dependent upon which second inputs 93 are enabled by therebalance approximation sequence control signals.

Converters 73, 90, and 91 may be substantially identical. Theattenuation provided by converters 90 and 91 may be understood byreference to corresponding explanation of converter 73.

Full adjust potentiometers 94, 95, and 96 correspond to tanks 21L, 21C,and 21R respectively and to full reference capacitors 100, 102, and 104respectively.

Liquid Measurements

If a liquid measurement corresponding to a particular tank is beingmade, the current provided by full reference circuitry 36 will, uponcompletion of a rebalance approximation sequence discussed below,balance the current corresponding to the capacitance added by the liquidto the probe 22 or dielectric sensor 23 being monitored. During such ameasurement, the current from full reference circuitry 36 is provided tothe input of the appropriate amplifier through a full referencecapacitor corresponding to the tank containing the probe or dielectricsensor of interest. The current provided by circuitry 36 is opposite inphase to the current provided by the probe 22 or dielectric sensor 23.

Accordingly, if a particular probe 22 in tank 21L is being monitored,the current provided to input 55L by full reference capacitor 100 willnull the current provided to input 55L by the capacitance of theparticular probe attributable to the liquid. Similarly, when themeasurement is being made within tank 21C or 21R, the current providedto input 55C or 55R respectively by full reference capacitor 102 or 104respectively will null the current provided by the particular probe ordielectric sensor capacitance attributable to the liquid.

Full Adjust Potentiometer 94, 95, and 96 Adjustment

As was previously indicated, during the initial set up of a system, eachfull adjust potentiometer (or other suitable voltage divider) is set(sized) to balance the current flowing through its corresponding fullreference capacitor with the current that would flow through the probe22 having the largest capacitance in the corresponding tank while theprobe is fully wetted.

The adjustment is accomplished in a manner similar to that used forempty adjust potentiometers 74, 75, and 76. Each full adjustpotentiometer is set so that the product of the full reference voltageprovided by amplifier 38 and the capacitance of the corresponding fullreference capacitor is equal to the product of the excitation signalvoltage provided by excitation signal generator 26 and the fully wettedcapacitance of the corresponding largest probe 22.

Accordingly, full adjust potentiometer 94 is set so that the product ofthe full reference voltage and the capacitance of full referencecapacitor 100 is equal to the product of the excitation signal voltageand the fully wetted capacitance of the largest probe 22 in tank 21L.Similarly, full adjust potentiometer 95 and 96 are set so that theproduct of the full reference voltage and the capacitance of fullreference capacitors 102 and 104 respectively are equal to the productof the excitation signal voltage and the fully wetted capacitance of thelargest probe 22 in tanks 21C and 21R respectively.

Resistors 94A, 95A, and 96A serve the same purpose as resistors 74A,75A, and 76A in empty reference circuitry 34. Similarly, thermistors94B, 95B, and 96B (FIG. 3A) serve to provide compensation to offset thefixed-temperature coefficients of full reference capacitors 100, 102,and 104 respectively.

Rebalance Approximation Sequence

Whenever excitation multiplexer 44 selects the largest probe 22 in atank for measurement, all second inputs 92 of full set D/A converter 90are enabled so that converter 90 provides the full reference voltagewithout substantial attenuation. During such a measurement, the currentflowing though the corresponding full reference capacitor will balancethe current flowing through the corresponding probe having the largestcapacitance while fully wetted.

However, if a measurement is being made on a probe 22 other than thelargest capacitance probe or on a dielectric sensor 23, the currentflowing through the corresponding full reference capacitor will belarger than the current corresponding to the fully wetted capacitance ofthe probe unless an appropriate attenuation is provided by full set D/Aconverter 90. Thus, whenever such a measurement is selected,microcomputer 25 provides appropriate attenuation control signals tosecond inputs 92, there being stored in a memory of microcomputer 25 afull set attenuation factor for each probe and dielectric sensor. Withthe appropriate attenuation enabled, the current provided to thecorresponding amplifier input by the applicable full reference capacitorwill null the current corresponding to the fully wetted capacitance ofthe probe 22 or dielectric sensor 23 of interest.

However, if a probe 22 is only partially wetted, or if a dielectricsensor 23 is immersed in a fuel of less than maximum dielectric constant(a partially wetted dielectric sensor 23 not being of interest), thecurrent provided through the corresponding full reference capacitor willbe too large to balance the current provided by the probe. Therefore,under such conditions, an additional attenuation is required in order toobtain a null at the input of the amplifier corresponding to the probe.This additional attenuation is obtained through rebalance D/A converter91 which, as previously indicated, is controlled by a rebalanceapproximation sequence control signals received through second inputs 93from microcomputer 25.

In setting rebalance D/A converter 91, microcomputer 25 uses asuccessive approximation program to generate the rebalance approximationsequence control signals. Through the rebalance control signals,microcomputer 25 first enables only the most significant bit (MSB) ofconverter 91. As is further explained under the discussion of nullthreshold detector 71, if the current thus provided through theapplicable full reference capacitor is larger than the current providedby the probe or dielectric sensor of interest, an output signal fromnull threshold detector 71 will be low, causing microcomputer 25 todisable the MSB. If the current provided by the applicable fullreference capacitor is smaller than the current provided by the probe ordielectric sensor of interest, the output signal from null thresholddetector 71 will be high, and microcomputer 25 will leave the MSBenabled.

This procedure is repeated for each of the remaining bits in rebalanceD/A converter 91. At the conclusion of these steps, the current at theinput of the applicable amplifier will be balanced to within theequivalent of one-half of the converter's least significant bit. Thestate of the converter's second inputs 93 (each input will be either alogic "1" or a logic "0") now defines a binary word. This digitalinformation represents the capacitance added by the fuel to the probe ordielectric sensor being monitored and is used by microcomputer 25 infuel volume and quantity determinations.

Reference Component Measurements

In addition to making liquid measurements on probes 22 and dielectricsensors 23 within tanks, full reference circuitry 36 is employed to makemeasurements on each of two reference components, a precision referenceresistor 45 and a precision reference capacitor 42.

As with the probes 22 and dielectric sensor 23 in each tank, referenceresistor 45 and reference capacitor 42 are each driven by excitationsignal generator 26, and each have a corresponding input amplifierthrough which measurements are obtained. Amplifier 50 corresponds toreference resistor 45 while amplifier 46 corresponds to referencecapacitor 42. Each of these amplifiers 50 and 46 are separatelyconnected to input multiplexer 60 which, as previously indicated,selects one amplifier at a time for obtaining a measurement.

Measurements made on reference resistor 45 and reference capacitor 42relate to system performance analysis. For example, by switching inreference resistor 45 and using null detection circuitry 68 to measurethe current driven through reference resistor 45 by the excitationsignal generator, the measurement obtained can be compared against aknown standard stored in microcomputer 25. If the appropriate value isnot obtained, it can be concluded that the excitation signal voltagelevel has shifted since a predetermined current through a precisionresistor will provide a predetermined voltage level. In this manner, aprimary check of the excitation signal level is made, and a secondarycheck of input multiplexer 60 and null detecting circuitry 68 isobtained. A test value related to whether the measured signal level iswithin predetermined limits of the known standard can be stored inmicrocomputer 25 for future reference. Storage for such a test valuecould comprise non-volatile memory 243 (FIG. 3F).

Similarly, after the null is obtained on the above measurement, thecurrent through reference resistor 45 is measured through contaminationmonitor 67 as further discussed in connection with the explanation ofthe contamination monitor. In this manner, a primary check of thecontamination monitor is made, and a secondary check of the excitationsignal level is obtained.

Further by switching in reference capacitor 42 and using null detectioncircuitry 68 to measure the current through reference capacitor 42, themeasurement obtained can be compared against a known standard stored inmicrocomputer 25. If the appropriate value is not obtained, it can beconcluded that the frequency of the exitation signal provided byexcitation signal generator 26 has shifted since a known current througha precision capacitor will provide a known frequency. In addition tobeing a primary check of the excitation signal frequency, this test isalso a secondary check of input multiplexer 60 and null detectingcircuitry 68. As in the case of measurements on reference resistor 45, atest value related to whether the measured signal level though thereference capacitor is within predetermined limits of the known standardcan be stored in microcomputer 25 for future use.

As with measurements obtained through amplifiers 55L, 55C, and 55R, if ameasurement is being made on reference resistor 45 or referencecapacitor 42, the current provided by full reference circuitry 36 to theamplifier input of interest will be equal in magnitude but opposite inphase to the current provided to the input by the component of interest.

Accordingly, when a measurement is being made on reference resistor 45,the current provided by full reference circuitry 36 to input 47 ofamplifier 50 will, upon completion of the rebalance approximationsequence, null the current provided by reference resistor 45 to input47. Similarly, if a measurement is being made on reference capacitor 42,the current provided by full reference circuitry 36 to input 41 ofamplifier 46 will null the current provided to input 41 by referencecapacitor 42. The currents provided to input 47 may be provided directlyfrom full reference circuitry 36 or as shown through a scaling resistor106.

In the system embodiment shown, reference resistor 45 receives theexcitation signal provided by excitation signal generator 26 through aswitching means 43 while reference capacitor 42 receives the excitationsignal directly from excitation signal generator 26.

Elimination of Empty Reference Circuitry 34

Empty reference circuitry 34 and its associated empty referencecapacitors 80, 82, and 84 can be eliminated from the present system.However, in doing so, the size and, therefore, the precision of D/Aconverters 73, 90, and 91 must be considered. In the system as disclosedhere, D/A converters 73, 90, and 91 are 10 bit units.

Eliminating empty reference circuitry 34 eliminates D/A converter 73which is part of that circuitry. Therefore, in order to eliminate emptyreference circuitry 34 while still maintaining the precision availablefrom the present system as disclosed, D/A converters more precise than10 bit units must be used for converters 90 and 91.

In addition, if empty reference circuitry 34 is eliminated, a storedvalue for the unwetted capacitance of each probe and dielectric sensorwould normally be used to determine the capacitance added to aparticular probe or dielectric sensor by liquid, the capacitance addedby the liquid being the total wetted capacitance less the dry orunwetted capacitance. Such use of a stored value would replace the useof stored empty set attenuation factors used as previously explained toset the attenuation of empty set D/A converter 73 in circuitry 34.

Circumstances leading to a decision to eliminate empty referencecircuitry 34 could involve, for example, having D/A converters withgreater precision than 10 bit units becoming less expensive or havinginsufficient space available on system printed circuit boards for emptyreference circuitry 34 and its associated empty reference capacitors.

If empty reference circuitry 34 is eliminated, the entire currentnecessary to balance the current provided by the probe 22 or dielectricsensor 23 of interest is provided by what is now full referencecircuitry 36.

Unwetted Sensor Calibration

In operation of the illustrated embodiment of the system, the dry orunwetted capacitance of each probe 22 and dielectric sensor 23 is storedin memory and used in setting empty set D/A converter 73 throughattenuation control signals. While each of these capacitance valuescould remain fixed throughout the life of the system, it is possible toprovide for unwetted capacitance variations which may occur. Forexample, the unwetted capacitance of each essentially identical probe ordielectric sensor typically varies in dependence on manufacturingtolerances and typically relates to stray capacitance in the system.

Accordingly, during initial system set up or during overhaul of asystem, the capacitance of one probe or dielectric sensor may vary fromanother essentially identical one. To avoid the effect of suchvariations, it is possible to have the present system read the dry orunwetted capacitance of each probe 22 and dielectric sensor 23 afterinstallation in the system and to store these values in storage meanssuch as non-volatile memory for use by the system. In this manner, aneven more precise system can be insured.

Excitation Signal Generator 26

In the embodiment illustrated, excitation signal generator 26 develops afrequency stable low distortion 18.75 KHz sine wave excitation voltagefrom a 3.0 MHz digital input signal, the 3 MHz signal being derived inmicrocomputer 25 (see FIG. 3F).

The 3 MHz input signal is buffered by a 54S04 inverter 111A and counteddown by a binary counter 111 to a frequency of 18.75 KHz. This frequencyis then buffered by another 54S04 inverter 111B. Amplitude control isprovided by a transistor 112, a zener diode 113, and associatedresistors.

The resulting frequency-stable, amplitude-controlled square wave isconverted to a low distortion sine wave by a two-pole, Chebechev activefilter 114. An amplifier 115 provides isolation for driving the probe 22and dielectric sensor 23 capacitive loads.

Binary counter 111 may comprise a 54S162 and a 54S161, the formerreceiving the 3 MHz signal through a pin 2, the latter receiving thesignal through a pin 10. Active filter 114 and amplifier 115 may eachcomprise an LF156.

Excitation Multiplexer 44

Excitation multiplexer 44 (FIG. 3B) is made up of 8 quad-CMOS analogswitches 116, 16 inverters 128, and a 4-to-16 line decoder 117comprising a 4-bit input latch and 6 inputs. Of these six inputs, fourinputs 48 are for receiving the input latch code, one input 48A is forreceiving an enable signal, and one input 48B is for receiving a strobesignal. These six inputs must be logic level compatible (+7.5 and -7.5volts) with excitation multiplexer 44 logic levels. In the embodiment asillustrated in FIG. 3B, this would require level shifting betweenmicrocomputer 25 and the excitation multiplexer.

In the preferred embodiment shown, quad switches 116 each comprise a4066B, inverters 128 each comprise a 4069, and line decoder 117comprises a 14514.

Line decoder 117 converts a 4-bit latched input code provided bymicrocomputer 25 into one of sixteen discrete outputs. These outputs areused to control the excitation signals which appear on the output ofswitches 116 at lines 53, 54, and 51. These lines provide the excitationsignal to selected probes 22, dielectric sensors 23, or load controlselect 52 respectively. An inhibit signal may be received by the enableinput in order to allow all 16 inputs to be deselected.

For each combination of inputs received by the 4-bit outputs latchwithin decoder 117, one of the 16 inputs on line decoder 117 isselected. In the embodiment shown, each of the 16 outputs is connectedto a quad switch 116 by a direct connection to either a control pin 113or a control pin 6 on the switch and through an inverter 128 to acontrol pin 5 or a control pin 12 on the switch respectively.

Each quad switch 116 comprises four switches used in pairs, each pairbeing used to comprise a single-pole double-throw switch connected to aprobe, a dielectric sensor, or load control select corresponding to eachtank.

In the preferred embodiment, for each channel of A/D electronics 24 and,therefore, for each excitation multiplexer 44 in a system, theexcitation multiplexer connects the excitation signal to one probe 22 ineach tank or to the dielectric sensor 23 in each tank or load controlselect 52. In addition, multiplexer 44 concurrently connects all otherprobes or dielectric sensors or load control select not receiving theexcitation signal to ground. For each channel of electronics 24, theexcitation signal or ground is received through a line 118 or a line 119respectively through an excitation and ground switching means 43discussed below.

Whenever line decoder 117 is enabled and one of its sixteen outputs isselected, the selected output switches from a low state to a high state,thereby providing a logic high to the corresponding quad switch 116, thelogic high being provided to either control pin 13 or control pin 6 onthe switch. The logic high is also passed through an inverter 128corresponding to the quad switch, thereby providing a logic low to theswitch at either control pin 5 or control pin 12 respectively.

Whenever a control pin 13 on a quad switch 116 receives a logic high,the switch within quad switch 116 corresponding to pin 13 is closed,thereby providing the excitation signal through the switch to thecorresponding probe 22, dielectric sensor 23, or load control select 52.At the same time, the switch corresponding to pin 5, which receives alogic low through an inverter 128, is opened, thus removing thecomponent from its connection to ground. Conversely, if a control pin 13receives a logic low and the corresponding pin 5 a logic high, thecorresponding component will be grounded.

Similarly, whenever a pin 6 receives a logic high, the switch withinquad switch 116 corresponding to pin 6 is closed, thereby providing theexcitation signal to the corresponding probe 22, dielectric sensor 23,or load control select 51, and the switch corresponding to pin 12simultaneously receives a logic low, thereby opening the switch anddisconnecting the component from ground. If a control pin 6 receives alogic low and the corresponding pin 12 a logic high, the correspondingcomponent will be grounded.

In this manner, the pair of switches corresponding to either pins 13 and5 or pins 6 and 12 operate as single-pole double throw switches toeither excite or ground each probe 22, dielectric sensor 23, or the loadcontrol select 52.

Excitation And Ground Switching Means 43

An excitation or ground switching means 43 may be included within aliquid gauging system comprising more than one channel of A/Delectronics 24 in order to permit application of the excitation andground signals from any of the system channels. Thus, if the excitationmultiplexer 44 in one channel should fail, it would be necessary to haveindependent control of whether ground or excitation signals are beingprovided through the multiplexer. Each excitation and ground switchingmeans 43 includes at least one switch for providing the excitationsignal through line 118 and at least one switch for providing groundthrough line 119.

In the configuration shown in FIG. 3E, switching means 43 comprises asingle 4066B quad switch 108 comprising four switches. Of the fourswitches, one is used to control application of the excitation signal toline 118, two are used in series to apply application of ground to line119, and one is not used.

Quad switch 108 is controlled through control signals received frommicrocomputer 25 through inputs 109. These three control inputs must belogic level compatible (+7.5 and -7.5 volts) with excitation multiplexer44. In the embodiment illustrated, this would require level shiftingthese three signals from the microcomputer. Upon receipt of appropriatecontrol signals through inputs 109, pins 1, 2, 3, and 4 within quadswitch 108 are connected, thereby connecting line 119 to ground 78,(although not always numbered or discussed, the same symbol as used forground 78 here is used for ground throughout this application).Similarly, upon receipt of appropriate excitation control signals, pins10 and 11 are connected, thereby providing to line 118 the excitationsignal received by quad switch 108 through pin 11.

Although excitation and ground switching means 43 is shown to comprise asingle quad switch 108, that switch providing two series switches forapplication of ground and one switch for application of the excitationsignal, it may be more appropriate to use separate chips for eachswitch. Using such an approach with each chip capable of independentlyproviding the necessary control, all switching capability will not belost in the event of failure of one chip.

Current-to-Voltage Input Amplifiers

As has been previously explained, eight input amplifiers, each of whichmay comprise an LF156, provide eight individual inputs to inputmultiplexer 60. Three of these amplifiers, 56L, 56C, and 56R, are usedfor liquid measurement inputs, amplifier 56L corresponding to tank 21L,amplifier 56C corresponding to tank 21C, and amplifier 56R correspondingto tank 21R. Two of the amplifiers, 50 and 46, relate to measurements onreference resistor 45 and reference capacitor 42 respectively. Theremaining three amplifiers 57 are used for load select control 52thumbwheel digit inputs.

In addition to providing a junction at which currents are nulled, aprimary function of each of the eight amplifiers is to convert inputcurrents, both capacitive and resistive, into a voltage for use in thenull detecting part of A/D electronics 24. In the embodiment shown, theamplifiers also provide a 180 degree signal phase shift and serve asisolation between the probes and sensors of different tanks.

As can be seen from FIG. 3C, capacitive feedback is used for capacitiveinputs and resistive feedback is used for resistive inputs. Suchfeedback maintains the required phase relationship between the inputcurrents received at the amplifier inputs and the signals provided bythe amplifiers. This phase relationship will be further discussed underthe discussion of quadrature filter 66 and quadrature filter drivers 40.

Input Multiplexer 60

The function of input multiplexer 60 (FIG. 3D) is to facilitate theselection of particular measurements. Multiplexer 60 comprises twoquad-BIFET switches 120 and 121 and a 4-to-16 line decoder 124 with a4-bit latch.

Quad switches 120 and 121 may each comprise a 11202. Each quad switch120 and 121 is used as four single-pole, single-throw switches.

Decoder 124, which may comprise a 14514, converts 4-bit coded inputcontrol signals received through four second inputs 59 into 8 discreetoutputs. Input 59A is used to receive an enable signal, and input 59B isused to receive a strobe signal. The eight discreet outputs are used toselect the particular tank or the component to be monitored.

Gain Scheduled Amplifier 61

At the same time that input multiplexer 60 selects a particular tank orcomponent for measurement, gain scheduled amplifier 61 selects one offour gain resistors 62, 63, 64, or 65 for use in providing appropriategain. The purpose of gain scheduled amplifier 61 is to maintain adequatesensitivity of the null detecting circuitry over a full range of inputsignal levels.

Amplifier 61, which shares some components with contamination monitor 67discussed below, comprises a quad-BIFET switch 123, a quad latch 125, adual 2-to-4 line decoder 126, and an inverting amplifier 127. As is alsodiscussed below, quad switch 122 is a part of contamination monitor 67and is not a direct part of gain scheduled amplifier 61.

Quad switch 123, which may comprise a 11202, serves to select one of thefour gain resistors 62, 63, 64, or 65.

Quad latch 125, which may comprise a 4042, holds a 4-bit word composedof two bits for input signal select and two bits for gain select. Thefour bit word is in binary code and is received through second inputs 58from microcomputer 25. Line decoder 126, which may comprise a 4555, isshown connected to quad latch 125 with 4 lines. Decoder 126 comprisestwo parts. One part uses two of the four connecting lines to obtain atwo bit binary code associated with gain scheduled amplifier 61. Each ofthe four combinations within this two bit code (00, 01, 10, and 11)correspond to selecting one of the four line decoder output pins (4, 5,6, or 7) applicable to the four gain resistors 62, 63, 64, and 65 withingain scheduled amplifier 61. Similarly, the other two connecting linesare used to obtain an alternate two bit binary code, each of the fourcombinations of the alternate code corresponding to one of the four linedecoder output pins (9, 10, 11, or 12) applicable to contaminationmonitor 67, the operation of which is further discussed below. Thus,within the four bit input signal held by quad latch 125, two bits areused by line decoder 126 to control the operating mode of contaminationmonitor 67 as further explained below, and two bits control the gain ofamplifier 61 through gain resistors 62, 63, 64, and 65. Input 58B ofquad latch 125 receives a strobe signal. Inputs 58A of decoder 126receive enable signals.

Amplifier 127, which may comprise an LF156, is used in conjunction withthe four gain resistors to provide appropriate gain. Since the input toamplifier 127 is connected to an inverting input pin 13, amplifier 127will invert signals amplified by it.

Null Detecting Circuitry 68

A section of null detecting circuitry 68 (FIG. 3E) has a primary purposeof detecting nulls at the input of the current-to-voltage inputamplifier corresponding to the measurement being made. As will beexplained further below, circuitry 68 comprises not only a nullthreshold detector 71 but also a quadrature filter 66 and a noise filter70. Quadrature filter drivers 40 and monostable trigger 32 (FIG. 3A)also relate to the operation of circuitry 68.

Quadrature Filter 66

Quadrature filter 66 comprises a demodulator 130 and an amplifier 146.In the embodiment shown, demodulator 130 comprises two NPN bipolartransistors, and amplifier 146 comprises an LF156. Together with drivers40, quadrature filter 66 filters the quadrature (resistive current)component of each liquid measurement signal in order to measure the truecapacitive value of the signal.

For good quadrature rejection and precise detection of liquidmeasurement signals, drive signals received at inputs 135 and 136 mustbe in phase (or 180 degrees out of phase) with the liquid measurementsignals received at input 137. In addition, the drive signals must havea 90 degree phase relationship with respect to the quadrature orresistive signal to be filtered.

FIG. 4 illustrates the relationships of various waveforms involved withthe operation of null detecting circuitry 68. Signal A in FIG. 4represents the sine wave excitation signal provided by output 30 ofexcitation signal generator 26. With respect to the remaining signals,it will be recalled that a primary purpose of amplifiers 56L, 56C, and56R is to convert a liquid measurement capacitive current signal into avoltage signal. After processing by gain scheduled amplifier 61, eachsuch voltage signal is received at input 137 to null detecting circuitry68.

As will be explained further below, whether a particular liquidmeasurement signal provided at input 137 to circuitry 68 is in phasewith signal A or 180 degrees out of phase with signal A depends uponwhether the capacitive current provided by a probe 22 or dielectricsensor 23 within a tank is greater than the current provided by acorresponding full reference capacitor.

If the current provided by the full reference capacitor is greater thanthe current provided by a probe or dielectric sensor, the voltage atinput 137 will be 180 degrees out of phase with excitation signal A asshown at signal C. This is the case since, as previously explained,excitation signal A is inverted in full reference circuitry 36, sincethe current-to-voltage input amplifiers are inverting amplifiers, andsince amplifier 127 is connected as an inverting amplifier.

Conversely, if the current provided by the probe or dielectric sensor isgreater than the current provided by the corresponding full referencecapacitor, the voltage signal provided at input 137 will be in phasewith excitation signal A as shown at signal B. This is the case sincethe current-to-voltage input amplifiers are inverting amplifiers andsince amplifier 127 is connected as an inverting amplifier.

With regard to the above explanation, it will be recognized that whenexcitation signal A passes through a probe, dielectric sensor, fullreference capacitor, or other capacitive load, the capacitive currentwill lead the capacitive voltage by 90 degrees, thus causing a 90 degreephase relationship between excitation signal A and the capacitivecurrent. However, after passing through the inverting current-to-voltageinput amplifiers having capacitive feedback, the 90 degree lead of thecapacitive current is eliminated. Therefore, for the sake of simplifiedexplanation, these 90 degree phase shifts were eliminated from theprevious 180 degree phasing discussions.

Quadrature Filter Drivers 40

Drive signals are received at inputs 135 and 136 (FIG. 3E) and arederived through operational amplifiers 131 and 132 within quadraturefilter drivers 40 (FIG. 2A). Amplifier 131, which may comprise a 119,provides input 136 with a drive signal D which is substantially in phasewith excitation signal A. Amplifier 131 is used in conjunction with aphase adjusting potentiometer 142. A phase adjustment throughpotentiometer 142 (FIG. 3A) is required since, in addition to the 90 and180 degree phase shifts previously discussed, there will be some phaseshift between excitation signal A and the liquid measurement signalsreceived at input 137. Accordingly, potentiometer 142 is adjusted toobtain optimum quadrature rejection characteristics.

Amplifier 132, which may also comprise a 119, operates as an inverterand provides at input 135 a drive signal E which is 180 degrees out ofphase with drive signal D.

In the operation of circuitry 68, whenever drive signal D received atthe base of transistor 141 is positive, transistor 141 turns on andconducts any signal received in leg 137A to reference or ground 78. Inthis manner, whenever transistor 141 is on, signals received at input137 are transmitted through leg 137B to pin 5 of amplifier 146. Pin 5 isan inverting input of amplifier 146.

Similarly, whenever signal E received at transistor 140 is positive, anysignal received in leg 137B will be conducted to reference or ground 78.Therefore, whenever transistor 140 is on, signals received at input 137will be transmitted through leg 137A to pin 6 of amplifier 146, pin 6being a non-inverting input to amplifier 146.

Accordingly, when signals are received at input 137, they will beinverted whenever transistor 141 is on but will not be inverted whenevertransistor 140 is on. Therefore, when signal C is received at input 137,a signal H will appear at output 147 to amplifier 146. As can be seen,signal H is a negative full-wave rectified voltage. Similarly, whensignal B is received at input 137, a signal I will appear at output 147.Signal I is a positive full-wave rectified voltage.

Consistent with the above, whenever the current provided by a fullreference capacitor is greater than the current provided by acorresponding probe or dielectric sensor, a negative full-wave rectifiedsignal H is provided at output 147 of amplifier 146. Conversely,whenever the current provided by a probe or dielectric sensor is greaterthan the current provided by a corresponding full reference capacitor, apositive full-wave rectified signal I appears at output 147.

Noise Filter 70

As was previously indicated, a noise filter 70 is incorporated withinnull detecting circuitry 68. In the embodiment shown, filter 70comprises an LF156. Filter 70 is incorporated to filter not only noiseper se but also low frequency fuel-slosh perturbations. Filter 70 ismechanized by means of a second-order bandpass active filter that has anatural frequency at approximately the 37.5 KHz frequency used for probeand dielectric sensor excitation. The amount of filtering that can beobtained through filter 70 is determined primarily by the systemsampling time requirements.

Null Threshold Detector 71

In conjunction with circuitry previously discussed, null thresholddetector 71 comprises a comparator 143, an AND gate 144, and amonostable multivibrator 145. Comparator 143 may comprise a 119, ANDgate 144 a 54LS08, and monostable multivibrator 145 a 54LS123.Monostable trigger 32 also relates to the operation of detector 71.

As was previously noted, liquid measurement signals at output 147 willeither be in the form of signal H or signal I. Likewise, since noisefilter 70 is incorporated to filter noise and fuel-slosh perturbations,liquid measurement signals appearing at the input to comparator 143 willalso be substantially in the form of either signal H or signal I.

In the embodiment shown, comparator 143 is set to a positive threshold,the value of which is set to satisfy system sensitivity requirements,typically at a threshold just above zero. Since the pin 10 input tocomparator 143 is an inverting input, when negative signal H appears atcomparator 143, it will be inverted and will trigger the comparator ifthe incoming signal H has sufficient amplitude. Conversely, whenpositive signal I is received at comparator 143, it will be inverted toa negative signal and will not trigger the comparator.

When comparator 143 is triggered by an incoming negative signal, it willprovide an output signal J at output 148. As long as comparator 143 isbeing triggered, output signal J will change states (low to high andback to low) at a repetition rate of 37.5 KHz, twice the 18.75 KHzfrequency of the excitation signal provided by excitation signalgenerator 26. However, whenever a positive signal is received by thecomparator or whenever the threshold of a negative signal isinsufficient to trigger the comparator, output signal J will not appear,and a low state will remain at output 148.

Monostable Trigger 32

In order to facilitate the operation of circuitry 68, a trigger signal K(FIG. 4) is generated by a comparator 151 within monostable trigger 32(FIG. 3A). In the embodiment shown, comparator 151 comprises a 119. Themonostable trigger further comprises an input 31 driven by excitationsignal generator 26. Since both trigger signal K and demodulator 130drive signals D and E are generated directly from output 30 ofexcitation signal generator 26, and since the phasing of signals D and Eis adjusted with potentiometer 142 as previously discussed, triggersignal K and demodulator output signal H are synchronous.

Trigger signal K and comparator output signal J are ANDED through ANDgate 144. When signal K and signal J are at AND gate input pins 4 and 5respectively, monostable vibrator 145 will be triggered by an outputsignal from AND gate 144, and a signal a monostable multivibrator output154 will change from a low state to a high state. As long as both signalJ and signal K are present at AND gate 144, the signal at output 154will remain high. However, if a trigger pulse from AND gate 144 is notreceived within three cycles of demodulator output signal H, a timingcircuit within monostable multivibrator 145 will time out, and thesignal at output 154 will change from the high state to the low state.

The two-state signal at output 154 can now be related to settingrebalance D/A converter 91 through the successive approximation sequencepreviously described (see discussion under full reference circuitry 36).When a step in the successive approximation sequence causes demodulatoroutput signal I to drop below the threshold of comparator 143 or causessignal H to appear, the signal appearing at comparator output 148 willgo to a logic low and will no longer provide signal J to AND gate inputpin 5, thus inhibiting the triggering of monostable multivibrator 145.Conversely, if a signal I having sufficient amplitude appears,monostable multivibrator 145 will be triggered and provide signal J.

Accordingly, for example, if in the preferred embodiment the currentprovided to a particular current-to-voltage amplifier input by a fullreference capacitor is larger than the current provided to the amplifierinput by the probe 22 or dielectric sensor 23 being monitored, theoutput signal at output 154 will be low. Conversely, if the currentgenerated through a probe or dielectric sensor and provided to thecorresponding amplifier input is greater than the current provided tothe amplifier input by the applicable full reference capacitor, thesignal at output 154 will be high.

In this manner, by monitoring the signal at output 154, microcomputer 25can appropriately adjust rebalance D/A converter 91 and thereby seek anull in order to establish the value of the measurement being taken.

Contamination Monitor 67

Contamination monitor 67 (FIG. 3D) may be used to check two parameters.First, it can be used to determine probe or dielectric sensorcontamination. The contamination measurement can be made at two levels,a first level indicating that the probe or sensor is becomingcontaminated but is still usable and a second level indicating that theprobe or sensor is no longer usable. In addition, contamination monitor67 can be used in conjunction with reference resistor 45 to determine ifthe amplitude of the excitation signal provided by excitation generator26 is above a predetermined level.

Contamination is directly proportional to the dissipation factor whichis the resistive component of the current provided by a probe ordielectric sensor divided by the capacitive component of the current.

Contamination monitor 67, which shares some components with gainscheduled amplifier 61, comprises a quad latch 125, a dual 2-to-4 linedecoder 126, a quad-CMOS switch 122, an amplifier 155, a comparator 156,an AND gate 157, and a monostable multivibrator 160. In the embodimentshown, quad latch 125 comprises a 4042, line decoder 126 a 4555, quadswitch 122 a 4066B, amplifier 155 an LF156, comparator 156 a 119, ANDgate 157 a 54LS08, and monostable multivibrator 160 a 54LS123. Recallthat quad switch 123 is related to gain scheduled amplifier 61 and isnot a direct part of contamination monitor 67.

A first threshold of comparator 156 is established with resistors 161and 162. Each of two additional resistors 164 and 165 facilitateselection of two additional thresholds for comparator 156. As will beseen from the following explanation, the operation of contaminationmonitor 167 is much like that of null threshold detector 71.

Controlling the mode of operation of contamination monitor 67 relates toselecting predetermined pins within quad switch 122. The selection ofthese pins is accomplished through quad latch 125 and line decoder 126as previously discussed in connection with gain scheduled amplifier 61.

In monitoring the amplitude of excitation signal generator 26, pin 5 ofquad switch 122 is selected. This selection connects pin 3 and pin 4 ofswitch 122 and causes the excitation signal received at pin 3 to betransmitted out pin 4 and received at pin 2 of amplifier 155.

Amplifier 155 then produces a half wave rectified output signalsynchronous with each cycle of the excitation signal. The half waverectified output signal provided by amplifier 155 is received at pin 4of comparator 156, the threshold of which is established by resistors161 and 162.

Whenever the amplitude of the half wave rectified signal received at pin4 of comparator 156 is sufficiently high, comparator 156 will provide anoutput signal similar to signal J (FIG. 4) but having half thefrequency. That comparator output signal is then received at pin 1 ofAND gate 157 and is ANDED with trigger signal K (FIG. 4) received at pin2 of AND gate 157.

Thus, whenever the amplitude of the excitation signal is sufficientlyhigh, there will be synchronous signals at pins 1 and 2 of AND gate 157which will then trigger monostable multivibrator 160.

Multivibrator 160 will then provide at an output 163 a signal having ahigh state to indicate that the amplitude of the sine wave beingprovided by excitation signal generator 26 is sufficient. A test valuerelated to whether the excitation signal has an amplitude above thepredetermined threshold may then be stored in microcomputer 25 forfuture reference. Storage for such a test value could comprisenon-volatile memory 243 (FIG. 3F).

Whenever a probe 22 or dielectric sensor 23 is being monitored forcontamination, the capacitive current at the input of the correspondingcurrent-to-voltage input amplifier is first nulled so that only theresistive (quadrature) component of the liquid measurement signalremains. Then, in order to monitor the resistive signal, the output ofgain scheduled amplifier 61 (at pin 14 of amplifier 127) is transmittedto pin 2 of amplifier 155. This is accomplished by selecting either pin6 or pin 12 of quad switch 122, each of these pins corresponding to oneof the contamination levels previously discussed. Selecting either pin 6or pin 12 also selects pin 13 of switch 122, thereby disconnecting pin 4from pin 3 so that the excitation signal is no longer received at pin 2of amplifier 155. The selection process also connects pin 1 and pin 2 ofswitch 122, thereby providing the signal from the probe or sensor to pin2 of amplifier 155.

Amplifier 155 then provides a half wave rectified output signalsynchronous with each cycle of the excitation signal. The half waverectified output provided by amplifier 155 is received at pin 4 ofcomparator 156.

During the measurement of contamination, the threshold of comparator 156is established not only by resistors 161 and 162 but also by eitherresistor 164 or 165. If pin 6 of switch 122 is selected, pin 8 and pin 9of the switch are connected, thus putting resistor 164 in parallel withresistor 162 and establishing a first threshold for monitoringcontamination. If pin 12 is selected, pin 10 and pin 11 are connected,thus putting resistor 165 in parallel with resistor 162 and establishinga second threshold for monitoring contamination.

Therefore, when monitoring contamination, if the resistive component ofthe liquid measurement signal is of sufficient amplitude to exceed theselected threshold of comparator 156, an output signal from comparator156 will be simultaneously ANDED with trigger signal C. AND gate 157will then trigger monostable multivibrator 160, thus causing monostablemultivibrator 160 to provide at output 163 a signal having a high stateand indicating that the contamination level corresponding to thethreshold has been reached.

Accordingly, for the embodiment disclosed here, reaching a thresholdduring the measurement of contamination will indicate either that aparticular probe or dielectric sensor is becoming contaminated but isstill useful or that the probe or sensor is becoming overly contaminatedand should no longer be used. As with the threshold measurementsdiscussed in connection with the excitation signal level, a test valuerelated to whether the quadrature signal has an amplitude above theapplicable threshold can be then stored in microcomputer 25 for futurereference. Storage for such a test value could comprise non-volatilememory 243 (FIG. 3F).

Lost Probe Recovery

If a fault is detected on probe 22 or other sensor (e.g. a capacitiveprobe having zero capacitance or becoming overly contaminated and nolonger useful), it is possible with the present system to recover asignificant portion of the error that would normally be introduced byloss of the sensor. As can be noted in FIG. 7, the additional errorcaused by a lost probe begins when the probe is first wetted andincreases to a maximum when the probe is totally immersed. The maximumadditional error that results is equal to the percentage of the fulltank liquid volume corresponding to the probe characterization which isoperative. For characterizations 221-226 plotted on FIG. 7, the maximumerror can be seen to range from approximately five to 20 percent.

In the present system, however, recovery from a lost probe can berelatively simple. For each probe in a tank, one or more sister probesmay be utilized to estimate the wetted sensing length of a failed probe.Various techniques are made possible in the present system becauseindividual wetted sensing lengths are known for all good probes.

As an example of a preferred approach to lost probe recovery in thepresent system two sister probes, S1 and S2, are identified for eachprobe in a tank. The two sister probes are used in estimating the failedprobe's wetted sensing length through the wetted sensing lengths of thetwo sister probes. With the length and wetted sensing length (WSL) ofeach probe available from microcomputer 25, the estimated wetted sensinglength of the lost probe (EST WSL_(PROBE)) can then be calculated usingthe formula EST WSL_(PROBE) =LENGTH_(PROBE) X (WSL_(S1)+WSL_(S2))/(LENGTH_(S1+LENGTH) _(S2)).

More sophisticated methods are possible. For example, three sisterprobes that are partially wetted may be used to define a plane throughthree wetted sensing points. The estimated wetted sensing length for afailed probe can then be found as the intersection of the probe with thepreviously defined plane.

Microcomputer 25

Microcomputer 25, which is interconnected with A/D electronics 24 asshown in FIG. 3F, may comprise a standard 8-bit microprocessor 240 suchas an Intel 8085A. Memory to store program instructions and tankcharacteristics data can be provided through memory means 241 comprisinga read only memory (ROM) such as an MK36000 or an electricallyprogrammable read only memory (EPROM) such as a 2732. For non-volatilememory 243, an NMOS electrically-alterable read-only memory (EAROM) suchas a General Instruments 2055 may be employed. Scratchpad memory may beprovided through a random access memory (RAM) 244 such as a 2114. Inputports 245 may comprise a 54LS244, and output ports 246 may comprise a54LS373. A 6 MHz crystal 247 may be used to derive the 3 MHz signalprovided to input 27 of excitation signal generator 26. Crystal 247 maycomprise a CTS Knight, Inc. MP060. Enable control 250 may comprise a54LS138 and other gates and flipflops such as a 54LS74 and a 54LS00. Inthe present system, microcomputer 25 comprises storage means anddetermining means.

Probes 22

As has been previously explained, probes 22 may be capacitance typesensors such as the one illustrated in FIGS. 5A and 5B or may be anytype of probe compatible with the present invention. In addition, probes22 may be replaced by any type of sensor for providing informationrelative to a liquid level or depth at a particular point in a tank. Forexample, probes 22 could be replaced with sensors which could determinethe liquid level by reflections from the liquid surface, the reflectionsbeing received at one or more locations from within or outside of thetank.

Each probe in the present system is individually monitored in order todetermine the wetted sensing length (WSL) or portion of the probe beingwetted by liquid. With the wetted sensing length of each probe known,the level, volume or quantity of fuel in the tank can be determined.

In the case of capacitance type probes, determining the wetted sensinglength of each probe is accomplished by measuring the capacitance of theprobe and using that capacitance and the measured dielectric constant ofthe liquid to compute the wetted sensing length. The capacitance of aprobe 22 will change due to both the portion of the probe length wettedby the liquid and the dielectric constant of the liquid. Accordingly,wetted sensing length may be computed using the formula WSL=A(C_(M)-C_(E))/(K-1), where C_(M) =measured probe wetted capacitance, C_(E)=empty or dry probe capacitance, K=liquid dielectric constant, and A=aconstant relating to probe geometry and size.

A liquid height-volume study is typically conducted during the design ofa particular system in order to select an optimum set of locations forthe minimum number of probes in each tank. For example, for the systemdescribed here, fourteen probes were selected for each wing tank.

As was previously indicated, it is no longer necessary with the presentsystem to use conventional capacitance type probes having the physicalcharacterization typically required in prior art systems. Instead, amuch simpler probe such as that shown in FIG. 5 may be used. The probeshown in FIG. 5 is substantially without physical characterization andhas an essentially uniform capacitance per unit length. (Physicallycharacterized probes may be used with the present system, however. Infact, prior art systems compatible with individual monitoring ofphysically characterized probes can be retrofitted with the presentsystem in order to provide multiple characterization of each probe,thereby significantly increasing the accuracy of such systems).Obviously, many variations are possible, including sensing means otherthan probes.

An advantage of the simplified type of probe illustrated in FIGS. 5A and5B is that it can be designed for a maximum commonality of parts betweenprobes and with dielectric sensor 23. In the embodiment shown, eachprobe comprises an outer tube or electrode 170 and an inner tube orelectrode 171. In aircraft fuel gaging applications, probes 22 typicallyrange in length from approximately 3 to 80 inches. In the embodimentshown, mounting of each probe is provided through two Delrin brackets,an upper bracket 166 and a lower bracket 167. Both brackets 166 and 167are shown to be captive on outer electrode 170. Upper bracket 166 may bedesigned to permit limited angular motion while lower bracket 167 may bedesigned to float vertically to provide for alignment tolerances in theaircraft mounting structure.

Inner electrode 171 is typically fabricated of stainless steel tubinghaving a 0.98 inch outer diameter by 0.015 inch wall thickness. Abracket 172 may be welded into the upper end of inner electrode 171 formaking electrical connections to a high impedance lead.

Inner electrode 171 is typically supported in outer electrode 170 by aTeflon pin 173 that is captured by upper mounting bracket 166. Electrodespacing may be controlled by Teflon spacers 174 positioned uniformlyalong the length of the probe. The capacitance effect of spacers 174 maybe adjusted for in computing the probe wetted sensing length.

In the preferred embodiment, outer electrode 170 provides a minimumspacing of 0.40 inch between electrodes 170 and 171. The tubing forouter electrode 170 may be of aluminum, typically 1.780 inches insidediameter by 0.020 inch wall thickness.

An anodized finish may be provided on outer electrode 170, and innerelectrode 171 may be passivated and coated with approximately 0.0005inch of polyurethane varnish.

A bracket 175 is shown riveted to outer electrode 170 for lead wirestrain relief supports and for low impedance leadwire termination via aterminal 177. A clamp 180 is typically used to support both the high andlow impedance leadwires which may be run through heat shrinkable tubingprovided near the terminal end of the leadwires.

Clamp 180, which may comprise a formed aluminum piece, is typicallysized on one side of a mounting screw 181 to fit the high impedanceleadwire and on the other side to fit the low impedance leadwire.Bracket 175 may also be formed to provide a channel 182 on each side ofmounting screw 181, each channel being formed to capture one of the twoleadwires.

In the embodiment shown, a strain relief support 183 is attached tooutput electrode 170 to provide additional support for the leadwires andto prevent direct strain on bracket assembly 175. Support 183 may be atwo piece molded plastic assembly with recesses to accept the twoleadwires.

Each probe 22 may also comprise an upper end cap 184 and a lower end cap185. Both end caps are typically fabricated of molded Delrin material.If used, the caps serve primarily to protect the probes and prevent themfrom contacting the top and bottom of the tank. Such contact mightoccur, for example, under conditions of tank flexing.

Lower end cap 185 typically encloses the end surface of outer electrode170 but not the inner diameter of electrode 171. Such constructionpermits a maximum rate of level change in the tank without materialdifference in fuel level between the tank and the sensor. It also helpseliminate a build-up of contaminants.

Upper end cap 184 is shown comprising an integrally molded cover 186that serves two purposes. The first purpose is to position and retainthe high impedance lead going to terminal 172. The second purpose is toprevent contaminants and condensation from entering the space betweenthe electrodes or the area of terminal 172. In the embodiment shown,cover 186 is mounted with an integrally molded hinge 187.

In order to maintain appropriate rate of level change of liquid withinthe probe, upper end cap 184 should be vented. The venting is preferablylocated on the vertical sides of the cap rather than in cover 186 sothat contaminants and condensation are prevented from entering theprobe.

Dielectric Sensor 23

Dielectric sensor 23 is a capacitance type sensor with the function ofproviding a measurement of the dielectric constant of the liquid.Because a liquid such as fuel in various tanks may vary in dielectricconstant due to variables such as mixture and temperature, the systemmay include a dielectric sensor 23 in each tank for optimum systemaccuracy.

Dielectric sensor 23 is typically located near the bottom of the tankbut above the level where water and contaminants will be a problem. Inthe preferred embodiment, the present system includes means to bypassthe measurement of dielectric sensor 23 when the fuel level is below alevel that would cause the sensor to become uncovered. When the lattercondition exists, the latest valid measurement of dielectric constantmay be used. If scratchpad data should be lost, a value of nominaldielectric constant stored in permanent data memory may be used.

Dielectric sensor 23 may be designed for maximum commonality of partswith probes 22. Electrical connections may be made in the same manner aswith probes 22 connections, and mounting provisions may be essentiallyidentical. In the embodiment shown, dielectric sensor 23 comprises aninner electrode 193 and second, third, and fourth electrodes 192, 194,and 195 respectively. Typically, the overall length of dielectric sensor23 is approximately 12 inches. As shown, mounting is provided throughtwo Delrin brackets, an upper bracket 190 and a lower bracket 191. Upperbracket 190 may permit limited angular motion and lower bracket 191 mayfloat vertically to provide for alignment tolerances in the aircraftmounting structure.

Inner electrode 193 may be identical in diameter and wall thickness to aprobe inner electrode 171. Inner electrode 193 may be stainless steeltubing having a 0.98 inch outer diameter and a 0.015 inch wallthickness. Second electrode 192 may be similar to probe outer electrode170 to provide a minimum spacing of 0.40 inch between electrodes 192 and193. The tubing of second electrode 192 may be aluminum having 1.780inch inside diameter by 0.020 inch wall thickness. Similar tubing may beused for third electrode 194 and fourth electrode 195, which may haveinside diameters of 2.61 inches and 3.44 inches respectively.

Electrode spacing is shown controlled by sets of Teflon spacers 196. Thecapacitance of spacers 196 may be adjusted for in computing fueldielectric constant.

An anodized finish is typically provided on the outer three electrodes192, 194, and 195, and inner electrode 193 may be passivated and coatedwith approximately 0.0005 inch of polyurethane varnish.

A bracket 197 may be riveted to second electrode 192 for a leadwirestrain relief clamp 200 and for a low impedance leadwire terminal 201.

In the embodiment shown, leadwire strain relief clamp 200 is formed ofaluminum and retained to bracket 197 by a screw 202. Bracket 197 may beformed with a channel 203 on each side of screw 202 to provide furthersupport for the leadwires. At the points of contact with clamp 202 andchannels 203, each leadwire is typically encased in a heat shrinkabletubing section provided near the terminal ends of the leadwires.

A high impedance leadwire terminal 204 is typically welded into theupper end of inner electrode 193 for making electrical connection to thehigh impedance leadwire.

Connections between the pairs of electrodes, inner electrode 193 tothird electrode 194 and second electrode 194 to fourth electrode 195,may be made by metallic spacers 205.

A strain relief support 206 is shown attached to second electrode 192 inorder to provide additional support for the leadwire assembly and toprevent direct strain on bracket 197. Support 206 is typically formed ofa two piece molded plastic assembly with recesses to accept the twoleadwires.

Each dielectric sensor 23 may be provided with an upper end cap 207 anda lower end cap 210, the end caps typically being of molded Delrin.

Lower end cap 210 typically will enclose the end surface of secondelectrode tube 192 but will not extend into the inner diameter ofelectrode 192. This permits a maximum rate of level change in the tankwithout a material difference in liquid level between the tank andsensor 23. It also helps eliminate a build-up of contaminants.

Upper end cap 207 is shown to include an integrally-molded cover 211that serves two purposes. The first purpose is to position and retainthe high impedance leadwire going to terminal 204. The second purpose isto prevent contaminants and condensation from entering the space betweenelectrodes or the area of terminal 204. In the embodiment shown, cover211 is secured with an integrally molded hinge 213.

A molded plastic cover 212 is also shown and is used to protect thespace between the outer three electrodes 192, 194, and 195 fromcontaminants and water condensation by enclosing the upper ends of theseelectrodes.

As was previously explained, the dielectric constant of the liquidmeasured by dielectric sensor 23 is used together with the measuredprobe capacitance to compute a wetted sensing length of each individualprobe.

The dielectric constant obtained through dielectric sensor 23 may alsobe used to determine liquid or fuel density. Knowingly fuel density, onecan determine fuel weight by multiplying density times fuel volume. Fueldensity may be calculated using a relationship between dielectricconstant and density as follows: D=(K-1)/[A+B (K-1)], where D is thefuel density, K is the fuel dielectric constant, and A and B areconstants used for a particular fuel type such as JP-4. As analternative to determining liquid or fuel density through dielectricsensor 23, the density may be determined by any other density sensormeeting system accuracy requirements. An example of an alternate densitysensor is disclosed in the previously-mentioned co-pending applicationentitled "Low Radiation Densitometer."

Software

Those skilled in the art will recognize that basic information theoryderived from logical principles provides that all information no matterhow complex can be represented by a collection of binary (yes or no)expressions. Within a microcomputer or other computer or digital device,such expressions are typically called "bits" and are typically stored ina memory in the form of "logical highs" each representing a logical "1"(e.g., "yes") and "logical lows" each representing a logical "0" (e.g.,"no"). The memory storing such "logical highs" and "logical lows" istypically an apparatus comprising a predetermined array of gates orswitches which are either opened or closed, an open switch typicallyrepresenting a "logical low" (essentially 0 volts) in that location anda closed switch typically representing a "logical high" (e.g., 5 volts)in that location.

Accordingly, as those skilled in the art will also recognize, it isfrequently semantical to distinguish between hardware and "software"since "software" normally is permanently stored in a hardware devicesuch as a read only memory (ROM), thereby becoming a permanent portionof the device circuitry. In fact, virtually all "software" in thepresent system is permanently embodied in such hardware.

Microcomputer Characterization

Historically, probes comprising conventional gaging systems have beenmanufactured with individual physical characterizations causing them tooutput predictable although non-linear capacitance readings when theyare wetted by fuel. These characterizations were typically calculatedfor a full set of all probes in a tank based on probe locations in thetank. Total capacitance of all probes in the tank was then related tofuel volume over the system range of tank pitch and roll attitudes.

Thus, in a typical prior art system, each probe contained one physicalcharacterization which was derived from data encompassing all specifiedattitudes and volumes to be covered by the system. In addition, allprobes in a tank were typically monitored together.

In contrast, the preferred embodiment of the present system permanentlystores in a memory at least one characterization for each probe, witheach probe being separately monitored for its own volume measurementcontribution to the total volume of liquid in the tank. Further, thesystem is compatible with multiple characterizations of each probe sothat, for example, each best fit of volume contribution data versuswetted sensing length can be related to specific portions of thespecified volumes to be covered. As a result, highly precise fuel volumemeasurement is greatly facilitated.

Characterization is carried out in the present system by storingcharacterization data or parameter in the memory of microcomputer 25,there being at least one set of characterization parameters for eachprobe. In the system as disclosed, the characterization is carried outby subdividing each probe into theoretical sections or section lengths230 (FIG. 8). Each particular section length relates to at least onecharacterization parameter which defines a relationship between a liquiddepth and a value related to liquid quantity, e.g., in the preferredembodiment, a percentage of tank volume occupied by liquid if theparticular section length is wetted by the liquid.

Many other variations are possible. For example, if the probes 22disclosed in this application were to be replaced with sensors which,without being immersed, determine the liquid level or depth fromreflections off the liquid surface, the level or depth measurementscould be divided into predetermined increments analagous to the probesection lengths 23 disclosed here. Each particular increment, if withinthe depth measurement, could relate to at least one characterizationparameter defining a relationship between a liquid depth and a valuerelated to liquid quantity.

Each characterization parameter is typically based on tank shape andvolume and on sensor location. In addition, each characterizationparameter may be based on a physical condition related to orientation orattitude of the liquid surface in the tank. Examples of such physicalconditions include gravity and acceleration vectors or forces,orientation or attitude of the tank, and orientation or attitude of theaircraft. Thus, in basing each characterization parameter on a physicalcondition related to orientation or attitude of the liquid surface inthe tank, each parameter could be based on the nominal orientation ofthe liquid surface in the tank or on one or more of the above examplesof physical conditions related to liquid surface orientation.

The computations necessary to determine the characterization parametersare typically performed on a large-scale, high-precision computer suchas the Honeywell H-6080. In the case of aircraft fuel tanks, tank shapeand volume data for such computations are typically obtained fromaircraft manufacturers.

In operation of the system as disclosed, when a probe 22 is monitored,microcomputer 25 determines the wetted sensing length of the probe andthen applies the appropriate characteristic data to obtain a percentageof tank volume contribution from the wetted sections. The sum of alltank units in a tank provides a measure of liquid volume as a percentageof total tank volume.

The present system is also compatible with multiple characterization ofeach probe. Such characterization may be understood by referring to FIG.7 which is a plot having a vertical axis covering from zero to 100percent of probe wetted sensing length and a horizontal axis depictingpercent of total tank volume. A dotted plot 220 in FIG. 7 represents afixed characterization probe typical of the prior art. The six solidplots, 221 through 226, are six characterizations for a single probe. Ofthe six characterizations, two characterizations (221 and 222) coveraircraft flight conditions with characterization 221 covering zerothrough 33 percent of full tank volume and characterization 222 covering33 through 100 percent of full tank volume. Characterizations 223through 226 cover aircraft ground conditions with characterization 223covering zero through 30 percent of full tank volume, 224 covering 30through 60 percent, 225 covering 60 through 82 percent, and 226 covering82 through 100 percent. Since a tank unit having a fixedcharacterization 220 is limited to a "best fit" of multiplecharacterizations 221 through 226, a resultant error 227 can besubstantial.

For each characterization 221 through 226, a plurality of points fromunwetted (zero percent wetted sensing length) to completely wetted (100percent wetted sensing length) may be stored as a set in a microcomputermemory lookup table 231 (FIG. 8). For wetted sensing lengths between the12 points, an interpolation may be made. A 16-bit interpolationmeasurement provides an effective 0.025 percent interpolation accuracy.

As can be seen from FIG. 7, each of the characterizations 221 through226 cover the full wetted sensing length (zero through 100 percent) ofthe particular probe being characterized. As can also be seen, eachcharacterization covers only a predetermined range of total tank volume.

In the system as disclosed, either flight or ground characterization isselected through aircraft air/ground relay 28. It should also be notedthat such use of flight or ground characterization is only one of manypossibilities of monitoring a physical condition related to anorientation that the liquid occupies in the tanks (in an aircraft, theshape and orientation of, for example, the wing tanks is affected bywhether the aircraft is in the air or on the ground.)

In addition, each lookup table 231 comprising a particularcharacterization has a range of data meeting system accuracyspecifications as well as data outside the specified range. Thus, onceeither flight or ground characterization is selected and computation isin progress, table 231 selections may be based on previously computedvolumes. If the last computed volume is beyond the accuracy range forthe selected characterization, the proper characterization (the properlookup table 231) is selected for the next computation. If there has notbeen a previously computed volume (e.g., after powerup) a default tableis selected for the first computation.

System Executive

A major task of the executive portion of the system software iscontrolling program sequence by considering logic conditions determinedpreviously and initiating required operations.

Upon application of power, a microcomputer reset signal forces a programcounter to a specific location from which a power on initializationsequence begins. Upon completion of system initialization, the system isplaced in a fuel weight computation mode and fuel weight computationbegins for each tank. After all tank computations have been made,indicators may be updated by a display program.

Computation of the percentage of total tank volume measured by eachprobe is outlined in FIG. 8. As was previously indicated, each probe 22is divided into theoretical sections 230. As was also previouslyindicated, volumes (expressed as a percent of total tank volume)corresponding to each section are stored in lookup tables 231 in memory,there being for each probe a lookup table for each characterization suchas 221 through 226.

During the process of determining the percentage of total tank volumemeasured by each probe, the wetted sensing length of each probe iscomputed (as was previously indicated under the discussion of probes 22)from the measured capacitance added to the probe by the fuel and fromthe dielectric constant of the fuel. With reference to the computedwetted sensing length for a particular probe, the section 230 wettedsection lengths are then summed, and corresponding section volumes aresummed.

When a total of section 230 wetted sensing lengths exceeds the computedwetted sensing lengths for a probe, an interpolation is made to find thewetted sensing length of a partially wetted section. The interpolationfactor is applied to the corresponding section volume in order todetermine the corresponding partial section volume attributable to thepartially wetted section. The partial section volume is then added tothe previous sum in order to arrive at the total tank volumecontribution corresponding to the particular probe. After filtering,each probe volume is then added to find total tank volume.

Because tank volume computed at this point is a percentage of full tankvolume, actual fuel volume is computed by multiplying the percentage offull tank volume times maximum tank volume. Total fuel weight may thenbe computed by multiplying fuel volume times density.

The computation of fuel volume and weight as described above isaccomplished in the present system by using several subfunction softwaremodules permanently embodied in memory means 241 (FIG. 3F). A programdesign language pseudo-code for the computation is listed in Table Ibelow.

                  TABLE I                                                         ______________________________________                                        DO FOR EACH TANK                                                              RESET TANK VOLUME (%) TO ZERO                                                 READ DIELECTRIC CONSTANT                                                      DO FOR EACH PROBE 22                                                          SET PROBE 22 VOLUME (%) TO ZERO                                               READ PROBE 22 CAPACITANCE                                                     COMPUTE WSL (WSL.sub.M)                                                       SET TOTAL WSL TO ZERO                                                         DO UNTIL TOTAL WSL GREATER THAN WSL.sub.M                                     ADD SECTION WSL TO TOTAL WSL                                                  ADD SECTION VOLUME (%) TO PROBE 22 VOLUME                                     ENDDO                                                                         COMPUTE WSL OF PARTLY WETTED SECTION                                          COMPUTE INTERPOLATION FACTOR                                                  ADD PARTIAL SECTION VOLUME (%) TO PROBE 22                                    VOLUME                                                                        FILTER PROBE 22 (%)                                                           ADD PROBE 22 VOLUME (%) TO TANK VOLUME (%)                                    ENDDO                                                                         COMPUTE TOTAL TANK VOLUME (GALLONS/LITERS)                                    STORE VOLUME                                                                  OBTAIN DENSITY                                                                COMPUTE TANK FUEL WEIGHT                                                      STORE TANK FUEL WEIGHT                                                        FILTER TANK FUEL WEIGHT                                                       STORE FILTERED TANK FUEL WEIGHT                                               ENDDO                                                                         ______________________________________                                    

In order to obtain each fuel dielectric reading and in order to readeach probe 22, appropriate multiplexer switches are selected. Inaddition, for each measurement, two 10-bit values representing empty andfull set adjustments are output to empty and full set D/A converters 73and 93 respectively. Microcomputer 25 then uses successive approximationlogic as previously described to output values to rebalance D/Aconverter 91 in order to appropriately null the current from the sensoror probe being monitored. The program design language pseudo-code forthis process is listed in Table II below.

                  TABLE II                                                        ______________________________________                                        OUTPUT MULTIPLEXERS 44 AND 60 SELECTOR                                        VALUES                                                                        OUTPUT EMPTY SET ADJUST TO D/A CONVERTER 73                                   OUTPUT FULL SET ADJUST TO D/A CONVERTER 90                                    OUTPUT GAIN AMPLIFIER 61 AND CONTAMINATION                                    MONITOR 67                                                                    SETTINGS TO LATCH 125                                                         DO UNTIL NUMBER OF BITS TESTED EQUALS 10                                      INSERT TEST BIT INTO APPROXIMATE VALUE                                        OUTPUT APPROXIMATE VALUE TO D/A CONVERTER                                     91                                                                            READ OUTPUT OF NULL THRESHOLD DETECTOR 71                                     IF OUTPUT IS HIGH                                                             KEEP TEST BIT                                                                 ELSE                                                                          DISCARD TEST BIT                                                              ENDIF                                                                         ENDDO                                                                         TEST FOR REASONABLENESS                                                       ______________________________________                                    

Filtering

In the present system, software may be used to filter individual probe22 readings, fuel weight values, and display values. Individual probe 22readings may be averaged with an average of previous readings for thatprobe. This running average helps smooth out extreme readings that canresult from such things as fuel slosh.

Fuel weight values may be filtered by using a weighted average. Forexample, a new fuel weight may be given a weighting factor of 0.25, andthe old fuel weight may be given a weighting factor of 0.75. Addingthese two quantities will give a weighted average for the fuel weightthat approximates a filter with a 3.5 second time constant. This valuemay then be processed through an algorithm to prevent least significantbit flicker on a display. This smoothed value becomes the displayedvalue and the old fuel weight value.

Data may be sent serially to a conventional indicator throughconventional drive electronics. A 16-bit format may be used, and thedata may be in binary coded decimal (BCD) form. One bit may be used forthe hundreds digit with four bits each used for the tens, units, and thetenths digits. The sixteen bits may be arranged as follows: the firstbit may always by a 1, the second bit may be a display bit (for dualdisplay indicators), the next thirteen bits may be data for theindicator, and the last bit may be a parity bit (e.g., odd parity).

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. In a liquid gaging system whereina value related to a liquid quantity in a tank is determined by thesystem based on signals from a plurality of sensors, the accuracy of thevalue being dependent upon the validity of the signals from the sensors,an apparatus for reducing error in the value introduced by anincapacitated sensor, comprising:the plurality of sensors, each sensorproviding a liquid measurement signal relating to a liquid depth at aparticular location in the tank; means for selectively monitoring theliquid measurement signal of each individual sensor, the means forselectively monitoring being connected to the sensors; means foridentifying at least one sister sensor corresponding to each sensor; andmeans for determining an estimated liquid depth corresponding to theincapacitated sensor by using the liquid measurement signal of the atleast one sister sensor corresponding to the incapacitated sensor andfor determining the value based on the liquid measurement signals andthe estimated liquid depth, the means for determining being connected tothe means for selectively monitoring and the means for identifying;whereby the value has less error associated with it than if an invalidliquid measurement signal from the incapacitated sensor were used todetermine the value.
 2. The apparatus of claim 1 wherein at least one ofthe sensors comprises a capacitive probe.
 3. The apparatus of claim 2wherein the capacitive probe has an essentially uniform capacitance perunit length.
 4. In a liquid gaging system wherein a value related to aquantity of liquid in a tank is determined by the system based onsignals from a plurality of probes, the accuracy of the value beingdependent upon the validity of the signals from the probes, an apparatusfor reducing error in the value introduced by an incapacitated probe,comprising:the plurality of probes for mounting in the tank, each probehaving a length, each probe providing a wetted sensor length signalrelated to the portion of the probe immersed in the liquid, the portionof the probe so immersed being a wetted sensing length of the probe;means for selectively measuring the wetted sensor length signal of eachindividual probe, the means for selectively measuring being connected tothe probes; means for identifying at least one sister probecorresponding to each probe; and means for determining an estimatedwetted sensing length of the incapacitated probe by using the wettedsensor length signal of the at least one sister probe corresponding tothe incapacitated probe and for determining the value based on thewetted sensor length signals and the estimated liquid depth, the meansfor determining being connected to the means for selectively measuringand the means for identifying; whereby the value has less errorassociated with it than if an invalid wetted sensor length signal fromthe incapacitated probe were used to determine the value.
 5. Theapparatus of claim 4 wherein:the apparatus further comprises means forstoring the length of each probe, the means for storing being connectedto the means for determining; and the means for determining comprisesmeans for determining an estimated wetted sensing length of anincapacitated probe using the length of the incapacitated probe and thelength of at least one sister probe.
 6. The apparatus of claim 4 or 5wherein at least one of the probes comprises a capacitive probe.
 7. Theapparatus of claim 6 wherein the capacitive probe has an essentiallyuniform capacitance per unit length.
 8. In a liquid gaging systemwherein a value related to a quantity of liquid in a tank is determinedby the system based on signals from a plurality of probes, the accuracyof the value being dependent upon the validity of the signals from theprobes, an apparatus for reducing error in the value introduced by anincapacitated probe, comprising:the plurality of probes for mounting inthe tank, each probe having a length, each probe providing a wettedsensor length signal related to the portion of the probe immersed in theliquid, the portion so immersed being a wetted sensing length of theprobe; means for selectively measuring the wetted sensor length signalof each individual probe, the means for selectively measuring beingconnected to the probes; means for identifying a first sister probe anda second sister probe corresponding to each probe; means for storing thelength of each probe; means for determining an estimated wetted sensinglength of the incapacitated probe using the length of the incapacitatedprobe and the length and wetted sensing length of the first sister probeand the second sister probe corresponding to the incapacitated probe andfor determining the value based on the wetted sensor length signals andthe estimated liquid depth, the means for determining being connected tothe means for selectively measuring, the means for identifying, and themeans for storing; whereby the value has less error associated with itthan if an invalid wetted sensor length signal from the incapacitatedprobe were used to determine the value.
 9. The apparatus of claim 8wherein at least one of the probes comprises a capacitive probe.
 10. Theapparatus of claim 9 wherein the capacitive probe has an essentiallyuniform capacitance per unit length.
 11. The apparatus of claim 8wherein the estimated wetted sensing length of the incapacitated probe(EST WSL_(PROBE)) is calculated using the formula EST WSL_(PROBE)=LENGTH_(PROBE) X (WSL_(S1) +WSL_(S2))/(LENGTH_(S1) +LENGTH_(S2)),LENGTH_(PROBE) being the length of the the incapacitated probe, WSL_(S1)and WSL_(S2) being the wetted sensing lengths of the first and secondsister probes respectively, and LENGTH_(S1) and LENGTH_(S2) being thelengths of the first and second sister probes respectively.